Download Unit 1 B

Unit 1 B
Newton's Laws of Motion
Classical Mechanics
Describes the relationship between the
motion of objects in our everyday world
and the forces acting on them
 Conditions when Classical Mechanics
does not apply

very tiny objects (< atomic sizes)
 objects moving near the speed of light

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Forces
Usually think of a force as a push or pull
 Vector quantity
 May be contact or field force

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Contact and Field Forces
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Fundamental Forces

Types





Strong nuclear force
Electromagnetic force
Weak nuclear force
Gravity
Characteristics



All field forces
Listed in order of decreasing strength
Only gravity and electromagnetic in mechanics
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Newton’s First Law

If no forces act on an object, it
continues in its original state of motion;
that is, unless something exerts an
external force on it, an object at rest
remains at rest and an object moving
with some velocity continues with that
same velocity.
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Newton’s First Law, cont.

External force


any force that results from the interaction
between the object and its environment
Alternative statement of Newton’s First
Law

When there are no external forces acting
on an object, the acceleration of the object
is zero.
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Inertia

Is the tendency of an object to continue
in its original motion
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Mass
A measure of the resistance of an
object to changes in its motion due to a
force
 Scalar quantity
 SI units are kg

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Newton’s Second Law

The acceleration of an object is directly
proportional to the net force acting on it
and inversely proportional to its mass.


F and a are both vectors
Can also be applied three-dimensionally
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Units of Force
SI unit of force is a Newton (N)
kg m
1N  1 2
s
 US Customary unit of force is a pound
(lb)


1 N = 0.225 lb
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Gravitational Force
Mutual force of attraction between any
two objects
 Expressed by Newton’s Law of Universal
Gravitation:

m1 m 2
Fg  G 2
r
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Weight

The magnitude of the gravitational
force acting on an object of mass m
near the Earth’s surface is called the
weight w of the object

Fg = m g is a special case of Newton’s
Second Law

g can also be found from the Law of
Universal Gravitation
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More about weight

Weight is not an inherent property of
an object


mass is an inherent property
Weight depends upon location
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Newton’s Third Law

If two objects interact, the force F12
exerted by object 1 on object 2 is equal
in magnitude but opposite in direction
to the force F21 exerted by object 2 on
object 1.

Equivalent to saying a single isolated force
cannot exist
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Newton’s Third Law cont.

F12 may be called
the action force and
F21 the reaction
force


Actually, either force
can be the action or
the reaction force
The action and
reaction forces act
on different
objects
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Some Action-Reaction Pairs

FN and FN’




FN is the normal
force, the force the
table exerts on the
TV
FN is always
perpendicular to the
surface
FN’ is the reaction –
the TV on the table
FN = - FN’
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More Action-Reaction pairs

Fg and Fg’



Fg is the force the
Earth exerts on the
object
Fg’ is the force the
object exerts on the
earth
Fg = -Fg’
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Forces Acting on an Object
Newton’s Law uses
the forces acting on
an object
 FN and Fg are acting
on the object
 FN’ and Fg’ are
acting on other
objects

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